Existing process control systems often employ control valves to control fluid flow though the process control system. Because control valves occasionally fail, it is desirable to perform periodic diagnostics on process control devices or process control components, such as the control valves, to determine the operability and performance of such devices. Determining the operability of a process control device may permit better scheduling of maintenance of the process control device, thereby decreasing failure occurrences and down time. This may result in increased efficiency, safety, and revenue. The process control systems may use various sensors and other measurement devices to observe characteristics of a process control device. For example, some existing control systems may use a digital valve controller to measure and collect data from various sensors on a control valve.
One diagnostic used to evaluate control valves is a valve signature test that measures the position of an actuator or actuator valve opening against an input to the valve, such as an actuator pressure or control signal. A graphical presentation of a signature graph may make it easier for plant operators to notice or detect changes in the characteristics of a valve that may indicate degradation in equipment, and thus, some control systems may implement valve maintenance software, such as AMS™ ValveLink®. software from Fisher Controls International LLC of St. Louis, Mo., to display signature graphs. Some valve characteristics that may be determined from a valve signature test may include, but are not limited to, valve friction, actuator torque, dead band and shutoff capability, and actuator spring rate and bench set.
For example, a valve signature test may be run when a control valve is new in order to benchmark the control valve's performance (e.g., valve manufacturer testing). One skilled in the art may understand that the valve signature test may record and/or trend the travel distance or position of the moveable element, such as a valve plug, in the control valve when opening and closing with respect to the applied actuating pressure for initiating such movement. As subsequent valve signature tests are performed on the control valve over time, the results of the signature tests may be reviewed with respect to previous tests to determine various characteristic changes, such as changes in actuator spring rate and valve friction or torque, to determine whether any degradation in performance or control of the control valve has occurred.
In addition to valve signature testing, control valves often need leak testing to determine when and if the valve is leaking, thus needing repair or replacement.
Some process control systems may have valve positioning devices (e.g., positioners) that both measure the actual position of a valve member and compare the actual position against a desired position. If the actual position and desired position differ from one another, the positioner adjusts the actual position to match the desired position. Because the positioner both measures the signal inputs into the valve actuator and the position of the valve member, software within the positioner (or in a computer operatively connected to the positioner) may compare the actual measurements to desired or baseline measurements to determine whether valve performance is degrading. Positioners may include leak testing capability.
However, less sophisticated process control systems may utilize control valves without positioners. Currently no simple, cost effective, devices exist that are capable of monitoring the performance of control valves, or testing for leaks, without positioners.